The present invention relates to optical devices and, more specifically, to optical devices that enable control of dispersion in optical communication systems.
The demand for greater bandwidth in optical communications is driving the fiber-optic telecommunications industry to explore technologies to achieve faster transmission speeds and increased capacity. The increase in bandwidth, however, is limited by a number of fundamental factors such as attenuation, noise and dispersion.1,2 In particular, dispersion is problematic because it distorts and/or broadens the optical pulses used to carry information through the optical communication system, thereby leading to data transmission error, especially in long haul and/or high speed systems.
Various attempts have been made to control or counteract dispersion in optical communication networks. For example, dispersion compensating fibers (DCFs) are available from companies such as Lucent Technologies/OFS and Corning to provide a negative dispersion across a specific operating band.1,2 However, since DCFs provide essentially constant negative dispersion, DCFs are generally useful only for dispersion correction at one wavelength at a time. That is, a series of DCFs are needed to control dispersion over the full range of wavelengths used in the optical communication system. Therefore, dispersion compensation solutions based on DCFs tend to be complicated and expensive.
Another approach to dispersion control is the use of fiber Bragg gratings.1,2 A fiber Bragg grating includes a chirped Bragg grating or a number of Bragg gratings designed to reflect different wavelengths all formed in a length of fiber so as to provide dispersion compensation on input light. Like DCFs, however, fiber Bragg gratings are limited in the range of wavelengths over which they are effective. Therefore, several gratings are needed to provide dispersion compensation over the optical communication wavelength range. Fiber Bragg gratings can also induce dispersion ripple, which leads to undesirable distortion of the optical signals.
Still other dispersion compensation schemes involve the use of all-pass filters.3-6 All-pass filters are optical filters designed to provide phase compensation without affecting the amplitude of input light.3 For example, in U.S. Pat. No. 6,289,151 B1, Kazarinov et al. (hereinafter, Kazarinov) describes an all-pass filter based on a number of ring resonators in a plurality of feedback loops. The all-pass filter of Kazarinov compensates for optical signal dispersion by applying a frequency-dependent time delay to portions of the optical signal in the feedback loops. The frequency-dependent time delay is provided by cascaded or series ring resonators, each of the ring resonators having a different phase. One problem arises with respect to the all-pass filter of Kazarinov, however, is submitted since a plurality of ring resonators and couplers are needed to provide dispersion compensation over the optical communication bandwidth. Also high manufacturing tolerances are required to ensure balanced performance of the device in compensating the dispersion of optical signal at a range of frequencies.
As another example of an all-pass filter, J. Ip in U.S. Pat. No. 5,557,468 (hereinafter, Ip) discloses a dispersion compensation device based on a reflective Fabry-Perot etalon.7 The all-pass filter of Ip includes a Fabry-Perot etalon including two reflectors. Each reflector includes a single uniform reflectance value that is different from the reflectance value of the other reflector so as to provide an input port and a separate output port for monitoring, for example, the frequency of the signal output of the all-pass filter. Again, the range of frequencies over which the all-pass filter of Ip is effective remains limited. Ip suggests the use of two or more Fabry-Perot etalons with dissimilar reflectivity characteristics and offset center frequency response, but it is submitted that the manufacturing tolerances for such a multi-stage cascaded device make the device impractical.
A Fabry-Perot etalon including a 100% reflectance mirror as one of its reflectors (also known as a Gires-Tournois interferometer) is also used as an all-pass filter. However, since the Fabry-Perot etalon generally provides an output in the form of a series of Gaussian peaks, it is difficult to manufacture a single stage Gires-Tournois ferometer exhibiting the desired phase response over a desired range of wavelengths.
Still another example of an all-pass filter for dispersion compensation is a thin film-based coupled cavity all-pass (CCAP) filter as discussed, for example, by Jablonski et al.8 The CCAP filter of Jablonski et al. is essentially a series of interference filters cascaded together. The CCAP filter of Jablonski et al. is similar to the aforedescribed Kazarinov approach in that the CCAP filter consists of two or more cavities disposed between reflectors and cascaded together to form a single filter. The thin film-based CCAP filter includes a plurality of alternating low index and high index thin films designed to form a stack of reflector sections separated by low index xe2x80x9ccavityxe2x80x9d sections. The thin film configuration allows the device to be compact compared to the use of a series of adjacent Fabry-Perot filters. However, the design of the thin film-based CCAP filter including more than two cavities is submitted to be mathematically problematic and, further, since the number of materials available for use as the low index and high index materials is limited, the filter is difficult to implement as a practical device.
The present invention provides an optical device for dispersion compensation which serves to reduce or eliminate the foregoing problems in a highly advantageous and heretofore unseen way and which provides still further advantages.
1. K. Slocum et al., xe2x80x9cDispersion Compensators,xe2x80x9d Wit SoundView Corp. Report, May 29, 2001.
2. J. Jungjohann et al., xe2x80x9cWill Dispersion Kill Next Generation 40 Gigabit Networks?xe2x80x9d CIBC World Markets Equity Research, Jun. 19, 2001.
3. R. Kazarinov et al., xe2x80x9cAll-Pass Optical Filters,xe2x80x9d U.S. Pat. No. 6,289,151 B1, issued Sep. 11, 2001.
4. G. Lenz et al., xe2x80x9cOptical Communication System including Broadband All-Pass Filter for Dispersion Compensation,xe2x80x9d U.S. Pat. No. 6,259, 847 B1, issued Jul. 10, 2001.
5. C. K. Madsen et al., xe2x80x9cIntegrated Optical Allpass Filters for Dispersion Compensation,xe2x80x9d OSA TOPS vol. 29, WDM Components, pp. 142-149.
6. G. Lenz et al., xe2x80x9cOptical Filter Dispersion in WDM Systems: A Review,xe2x80x9d OSA TOPS vol. 29, WDM Components, pp. 246-253.
7. J. Ip, xe2x80x9cChromatic Dispersion Compensation Device,xe2x80x9d U.S. Pat. No. 5,557,468, issued Sep. 17, 1996.
8. M. Jablonski et al., xe2x80x9cThe Realization of All-Pass Filters for Third-Order Dispersion Compensation in Ultrafast Optical Fiber Transmission Systems,xe2x80x9d Journal of Lightwave Technology, vol. 19, no. 8, pp. 1194-1205, August 2001.
As will be disclosed in more detail hereinafter, there is disclosed herein an optical device for receiving input light and for acting on the input light to produce output light. The optical device includes a first reflector and a second reflector supported in a spaced-apart, confronting relationship with the first reflector such that the input light received by the optical device, at least potentially, undergoes multiple reflections between the first and second reflectors. At least a selected one of the first and second reflectors is configured to subject each one of a plurality of different portions of the input light to one of a plurality of different reflectance values to produce an emitted light passing through at least the selected reflector in a way which is combinable to generate the output light.
In another aspect of the invention, there is disclosed a dispersion compensation module including the aforedescribed optical device.
In still another aspect of the invention, a method for use in an optical device for receiving input light and for acting on the input light to produce output light is disclosed. The method includes the steps of supporting a first reflector and a second reflector in a spaced-apart, confronting relationship and configuring the first and second reflectors such that the input light received by the optical device, at least potentially, undergoes multiple reflections between the first and second reflectors. The method also includes the step of configuring at least a selected one of the reflectors to include a plurity of different reflectance values. The method further includes the step of subjecting a plurality of different portions of the input light, during the multiple reflections, to a plurality of different reflectance values at a selected one of the reflectors to produce an emitted light passing through the selected reflector in a way which is combinable to generate the output light.